Giant clams at risk

11 December 2017 | Research
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Tridacna gigas are the world’s largest giant clam species and the most threatened mollusc (Photo: Neo Mei Lin)

A comprehensive worldwide study of the status of giant clams by an international team of marine scientists, led by Dr Neo Mei Lin from the Tropical Marine Science Institute at NUS and Associate Professor Peter Todd from NUS Biological Sciences, has revealed that giant clams are facing a higher risk of extinction; the largest known species in particular.

Exploitation of giant clams goes back to the mid-19th century, when they were harvested for food and the curio trade. In the last century, habitat loss, reef degradation, as well as overfishing for food and shell craft have contributed to their depletion. These giant clams, predominately found in the Indo-Pacific region, can be considered engineers of the marine ecosystem. They build and shape reefs, filter water, and provide food for fellow reef creatures. Their dwindling populations and their threatened status have driven scientists to examine the clams’ global distribution and develop measures for conservation.

Focusing on the 12 currently recognised species of giant clams, the scientists reviewed all available data on the clams’ classification and global distribution, their exploitation and the laws that protect them, as well as the impact of the harvest rates on wild populations and outcomes of past and ongoing conservation programmes. Additionally, fellow marine scientists from around the world were recruited to assess and cross-examine the information.

According to findings by the marine scientists, the only truly gigantic species — Tridacna gigas — is facing the highest threat of extinction. This species can grow up to one metre long and weigh up to 300 kilograms, resulting in the most substantial meat mass for consumption. They are also easily harvested directly from the reef bed. In 26 of the 31 sites where natural wild populations were known to be present, Tridacna gigas were either severely depleted or could no longer be found.

In contrast, the smallest giant clam species, Tridacna crocea, is the least threatened and remains relatively abundant in the Indo-Pacific area despite ongoing fishing activities. This is likely due to its small size — at most 15 centimetres — and its burrowing behaviour which makes it difficult for fishers to remove them from the reef bed.

“We urgently need to step up on our conservation efforts to prevent the larger giant clam species from going extinct. The trends are worrying, so while there may be more of the smaller clam species now, it is only a matter of time before they become endangered too,” said Dr Neo, who is also from NUS Biological Sciences.

We urgently need to step up on our conservation efforts to prevent the larger giant clam species from going extinct. The trends are worrying, so while there may be more of the smaller clam species now, it is only a matter of time before they become endangered too.

In 2011, the NUS marine scientists began a restocking project of the fluted giant clam, Tridacna squamosal, in Singapore. This was partly encouraged by the achievements of the University of Philippines’ Marine Science Institute’s 20 year-old programme which cultured Tridacna gigas to restore the depleted populations and recently saw the appearance of new baby clams near the parent clams.

“The success by the Philippines team provides evidence that, given enough time and local protection, restocked clams can produce local juvenile recruitment. We hope to have similar success while restocking the fluted giant clam, Tridacna squamosa, in Singapore. While this species is not as threatened as Tridacna gigas, its numbers in Singapore are very low and the population is probably not currently self-sustaining,” said Assoc Prof Todd.

Having successfully spawned healthy fluted clam juveniles, the team transplanted about 250 hand-reared Tridacna squamosal onto various reefs among the Southern Islands in 2016. Their growth and survivorship continue to be assessed.

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Cultured Tridacna squamosa that were transplanted to reefs in the restocking project led by NUS marine scientists (Photo: Neo Mei Lin)

“I hope that by highlighting the threat that giant clams face, and demonstrating the success of current conservation measures, we can encourage further restocking efforts and provide a greater push for their conservation,” Dr Neo added.

The study, funded by the National Parks Board of Singapore, took place from 2014 to 2016. It was recently published in leading marine journal Oceanography and Marine Biology: An Annual Review.